Bulk optic pick-up heads which are used to retrieve information stored on optical disks are well known. The principles of operation of such devices are briefly as follows. Polarized light from a laser diode is collimated and sent through a polarization beam splitter (PBS) and a quarterwave plate. Circularly polarized light is focused onto the information bearing surface of the optical disk by an objective lens. Information is encoded in the form of pits and land areas which are located along tracks. As the disk spins, the intensity of the reflected light is modulated by reflection off of these pits and land areas. This modulation is ultimately sensed and used to generate the data signal. Furthermore, the distribution of the reflected light is modified by motion of the data tracks due to disk runout and is ultimately used to generate the tracking error signal (TES). The reflected light is then re-collimated by the objective lens. Because the degree of recollimation is directly related to the displacement of the reflecting surface of the optical disk from nominal focus as the disk rotates, measurement of this collimation is used to generate the focus error signal (FES). The sense of circular polarization is reversed upon reflection off of the optical disk so the encoded light becomes linearly polarized in a direction perpendicular to the initial polarization state upon passage through the quarterwave plate a second time. The state of polarization of this light is such that it is reflected by the PBS to a detector section which retrieves TES, FES, and the data signal. TES and FES are amplified and are used to drive electro-mechanical tracking and focus actuators, respectively, which are attached to the objective lens. These closed-loop servo systems maintain both tracking and focus as the disk spins.
Both partial, as well as, fully integrated optic heads have been recently proposed. In these devices, the conventional optical and electromechanical elements of bulk optical heads are replaced at least in part by planar waveguide optical and electro-optical elements, respectively. These integrated optic devices are intrinsically lower in mass and smaller in volume and form-factor than are the bulk optic counterparts and can be mass produced using IC-type processing techniques. Hence, they offer the possibly of improved performance and lower manufacturing costs compared to conventional pick-up heads. On the other hand, integrated optic heads must provide one or more of the key components which define an optical pick-up head; namely, a) a source of coherent light radiation which is focused into a diffraction-limited spot on the information-bearing surface of the optical disk, b) a means of retrieval of the modulated light returning from the disk, c) isolation of the modulated light returning from the optical disk from the source, d) means of sensing the TES, FES, and data signal, e) actuator means for correcting focus and tracking errors due to disk motion, and in the case of a read/write head f) a means of modulating the source intensity during the write operation.
Partially integrated optic heads incorporating acousto-optic light deflection as a tracking actuator means are revealed in U.S. Pat. No. 4,802,153 and by Arimoto et al. Waveguide Optical Deflector for an Optical Disk Tracking Actuator Using a Surface Acoustic Wave Device, Applied Optics, Jan. 10, 1990, pp. 247-250. Unfortunately, these devices both require relatively high power rf generators to drive surface acoustic wave (SAW) transducers. Additional disadvantages of these devices include relatively slow response times for the SAW deflection and rf noise generation. These devices are partially integrated in that they do not incorporate means for detection of TES, FES, data signal, or tracking and focus actuation. Consequently, they require additional bulk optical and mechanical elements to form a complete optical pick-up head.
Fully or near-fully integrated pick-up heads have been disclosed in Japanese Patent Laid-open No. 263350 (Dec. 26, 1985) and in U.S. Pat. No. 4,991,919. These devices are both characterized by two separated curved and blazed gratings. One grating couples the light out of the planar waveguide and focuses it onto the optical disk without the need of an external bulk objective lens. The light is reflected off of the disk at an angle such that when it returns to the planar waveguide, it is incident on the second grating which couples it into the waveguide. The blazing eliminates the possibility of light being coupled into waveguide substrate modes. The device disclosed in the former patent has the disadvantage that the spot quality produced by the focus grating output coupler is extremely sensitive to laser diode wavelength shift due to the dispersive nature of the focus grating coupler. The device described in the latter patent has the disadvantage that it does not provide for source isolation. The latter device also is complicated by the requirement for a unique polarizer which is capable of converting linear polarization to concentrically circular polarization. Neither device allows for electro-optic deflection or modulation.
European Patent Publication No. 0,174,008 discloses a fully integrated optical head based on an electro-optic waveguide such as single crystal LiNbO.sub.3. Electro-optic focus and tracking actuator functions are provided by shaped surface electrode structures. This device suffers the disadvantage that the surface electrode structures are necessarily far apart. Given the magnitude of the electro-optic coefficient of materials such as LiNbO.sub.3, this implies that unrealistically large voltages are required to obtain fringing fields in the electro-optic waveguide region between these surface electrodes large enough to cause appreciable deflection. This device concept also suffers the same difficulties mentioned above in that the focused spot quality on the optical disk is extremely sensitive to laser diode wavelength drift and the dispersive nature of the focus grating coupler.
The use of nonlinear optical materials in laser cavities to achieve conversion of stimulated radiation at the fundamental wavelength to radiation at the second harmonic wavelength is well known. The nonlinear optic (NLO) materials usually used in such applications are inorganic single crystals such as potassium dihydrogen phosphate (KDP), potassium titanyl phosphate (KTP), ammonium dihydrogen phosphate (ADP), lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3). Such devices are usually bulky and expensive to fabricate. Accordingly, it will be appreciated that it would be highly desirable to have a compact optical read/write device with a laser diode structure that incorporates NLO material and generates visible radiation by second harmonic generation (SHG) of the fundamental wavelength.